It sounds to me like the governor for the engine isn't working properly or smoothly. It might be something as simple as the mechanism binding or being dirty. It could very well also be the carburetor needing to be cleaned or some other general small engine repair/adjustment.
Pretty much any small engine repair place should understand the engine (if you don't feel up to checking it yourself); they're pretty much bog standard small engines, such as would be used on lawn mowers or whatever. Parts availability may be a bit limited for the Harbor Freight units, though they at least tend to have some support available.
